As an adsorbent used in CO.sub.2 gas separation and recovery by the PSA method, carbon molecular sieves, zeolite molecular sieves, activated charcoal and the like are known. However, generally, when water is accumulated in an adsorbent, in addition to a decrease in the amount of adsorbed CO.sub.2 gas and the separation capacity, various problems such as powdering of the adsorbent and the like are caused, which results in difficulty in recovery of the CO.sub.2 gas in high yield and high purity. Then, many attempts for removal of water in a raw material mixed gas have been proposed.
For example, JP-A 62-136222 proposes a process wherein a gas cooler or refrigerating dryer or the like is used as a dehumidification apparatus in order to bring an amount of water in a combustion exhaust gas to a degree of not higher than 20.degree. C. as reduced in terms of the dew point.
Also, JP-A 1-172204 proposes a process for preventing water from accumulating in a zeolite molecular sieve adsorbent wherein alumina gel as a desiccant is packed at a lower part of the zeolite molecular sieve adsorbent, that is, at the raw material mixed gas inlet side, water in a raw material combustion exhaust gas is removed with this desiccant and then CO.sub.2 gas separation is carried out at the upper part of the zeolite molecular sieve adsorbent.
On the other hand, as another process for removal of water, JP-A 1-108106 proposes a process wherein water in the raw material mixed gas is removed by pre-treating the raw material mixed gas according to a PSA method using a synthesized zeolite as an adsorbent before it is fed into a PSA apparatus.
Since the above-described various adsorbents, in particular, zeolite molecular sieves have especially high adsorbability of water and the water once adsorbed is difficult to be desorbed, water tends to accumulate in the adsorbent. Therefore, in the case of zeolite molecular sieves, more attention must be paid to the prevention of water accumulation. In order to solve this problem, the above-described JP-A 1-108106 discloses a process wherein PSA for the removal of water is provided as the pretreatment in addition to a CO.sub.2 gas PSA main apparatus. However, in this process, it is unavoidable that the plant cost becomes higher, the operation procedures become complicated and the running cost also becomes higher.
Although the process described in JP-A 1-172204 uses alumina gel as a desiccant, the desiccant must be used in a large amount. Further, since the regeneration of the alumina gel which has absorbed water is carried out with a described gas (CO.sub.2 gas), the desorbed CO.sub.2 gas is consequently accompanied with water in a raw material mixed gas. Therefore, it is necessary to provide a vacuum pump for desorption having a considerably larger capacity to bring a gas corresponding to CO.sub.2 gas and water vapor to evacuation. Furthermore, since much water drainage is formed at a CO.sub.2 gas recovery line following the vacuum pump, corrosion-resistant materials are required in order to prevent corrosion. Furthermore, since the drying of CO.sub.2 gas per se is required, there is such a problem that a dehumidification apparatus is also required in a post-treatment process.
Accordingly, in a process for separation of CO.sub.2 gas by a PSA method using zeolite molecular sieves as an adsorbent, for example, in the case where CO.sub.2 gas is recovered from a mixed gas containing water such as hot-blast stove exhaust gas, combustion exhaust gas or the like, it is desired to develop an excellent process for removal of water in the mixed gas while minimizing the above-described problems. In addition, a process for recovering CO.sub.2 gas in an exhaust gas as high purity CO.sub.2 using the exhaust gas as a raw material gas is a noticeable technique from the viewpoint of prevention of the recent green house effect of the earth.